A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case of a transistor included in a large display device, it is preferable to use amorphous silicon, which can be used to form a film on a large substrate with the established technique. In the case of a transistor included in a high-performance display device where a driver circuit and a pixel circuit are formed over the same substrate, it is preferred to use polycrystalline silicon, which can form a transistor having high field-effect mobility. It is known that polycrystalline silicon can be formed as a result of heat treatment at high temperatures or laser light treatment on amorphous silicon.
In recent years, transistors including oxide semiconductors (typified by an In—Ga—Zn oxide) have been actively developed.
Oxide semiconductors have been researched since early times. In 1988, it was disclosed to use an In—Ga—Zn oxide crystal for a semiconductor element (see Patent Document 1). In 1995, a transistor including an oxide semiconductor was invented, and its electrical characteristics were disclosed (see Patent Document 2).
In 2013, one group reported that an amorphous In—Ga—Zn oxide had an unstable structure in which crystallization is induced by irradiation with an electron beam (see Non-Patent Document 1). According to the report, no ordering was observed with a high-resolution transmission electron microscope in the amorphous In—Ga—Zn oxide formed by the group.
In 2014, a transistor including a crystalline In—Ga—Zn oxide that has more excellent electrical characteristics and higher reliability than a transistor including an amorphous In—Ga—Zn oxide was reported (see Non-Patent Document 2, Non-Patent Document 3, and Non-Patent Document 4). These documents reported that a grain boundary was not clearly observed in an In—Ga—Zn oxide including a c-axis aligned crystalline oxide semiconductor (CAAC-OS).
As a kind of polymer crystal structure, a concept of “paracrystal” is known. A paracrystal seemingly has a trace of crystal lattice; however, compared with an ideal single crystal, the paracrystal has a distorted crystal structure (see Non-Patent Document 5).